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In JavaScript, regular expression (RegExp) patterns define the structure and rules for matching strings. A pattern can include literal characters, metacharacters, character classes, quantifiers, assertions, groups, alternation, and flags to specify exactly what to match and how.
Literal patterns match exact characters in a string.
const str = "Hello World";
console.log(/Hello/.test(str)); // true
console.log(/World/.test(str)); // true
console.log(/world/.test(str)); // false (case-sensitive)
Case-insensitive matching: Use the i flag.
console.log(/world/i.test(str)); // true
Character classes match any one character from a set:
console.log(/[aeiou]/.test("hello")); // true (matches 'e')
console.log(/[0-9]/.test("123")); // true (matches '1')
console.log(/[^0-9]/.test("123")); // false (no non-digit)
[a-zA-Z0-9] matches letters or digits.\\d → any digit [0-9]\\w → any word character [a-zA-Z0-9_]\\s → any whitespaceExample:
console.log("a1b2".match(/[a-z]\\d/g)); // ["a1", "b2"]
Metacharacters define special matching rules:
| Metacharacter | Meaning |
|---|---|
. | Any character except newline |
^ | Start of string/line |
$ | End of string/line |
* | 0 or more repetitions |
+ | 1 or more repetitions |
? | 0 or 1 occurrence / non-greedy |
() | Groups pattern and captures match |
[] | Character class |
{n,m} | Repetition between n and m |
| ` | ` |
Examples:
const str = "cat or dog";
console.log(/cat|dog/.test(str)); // true
console.log(/c.t/.test("cot")); // true
Quantifiers specify how many times a part of the pattern repeats:
console.log(/a{2,4}/.test("aaa")); // true (2 to 4 'a')
console.log(/a*/.test("")); // true (0 or more 'a')
console.log(/a+/.test("a")); // true (1 or more 'a')
console.log(/a?/.test("")); // true (0 or 1 'a')
Greedy vs Lazy Matching:
const str2 = "<p>one</p><p>two</p>";
console.log(str2.match(/<p>.*<\\/p>/)); // greedy → matches both paragraphs
console.log(str2.match(/<p>.*?<\\/p>/g)); // lazy → matches each paragraph separately
Assertions match positions without consuming characters:
| Assertion | Meaning |
|---|---|
\\b | Word boundary |
\\B | Non-word boundary |
^ | Start of string/line |
$ | End of string/line |
(?=...) | Positive lookahead |
(?!...) | Negative lookahead |
(?<=...) | Positive lookbehind |
(?<!...) | Negative lookbehind |
Examples:
console.log(/\\bcat\\b/.test("the cat sat")); // true
console.log(/(?<=\\$)\\d+/.test("$100")); // true
( … ) → Capturing group for backreferences or replacements(?: … ) → Non-capturing groupExamples:
console.log("abcabc".match(/(abc)\\1/)); // ["abcabc", "abc"]
console.log("abc def".match(/(?:abc) def/)); // ["abc def"]
Matches one of multiple patterns:
console.log("cat".match(/cat|dog/)); // ["cat"]
Flags modify how patterns behave:
| Flag | Meaning |
|---|---|
g | Global search (all matches) |
i | Case-insensitive |
m | Multiline (^/$ match lines) |
s | Dot matches newline |
u | Unicode mode |
y | Sticky match (from current position) |
Example:
console.log("Cat cat".match(/cat/gi)); // ["Cat", "cat"]
Email Validation:
const emailPattern = /^[\\w.-]+@[\\w.-]+\\.\\w+$/;
console.log(emailPattern.test("[email protected]")); // true
Phone Number (XXX-XXX-XXXX):
const phonePattern = /^\\d{3}-\\d{3}-\\d{4}$/;
console.log(phonePattern.test("123-456-7890")); // true
Date Validation (DD/MM/YYYY):
const datePattern = /^\\d{2}\\/\\d{2}\\/\\d{4}$/;
console.log(datePattern.test("13/10/2025")); // true
Extract URLs:
const urlPattern = /https?:\\/\\/[^\\s]+/g;
console.log("Visit <http://example.com>".match(urlPattern)); // ["<http://example.com>"]
Match Repeated Words:
const repeatPattern = /\b(\w+)\b\s+\1\b/;
console.log("hello hello world".match(repeatPattern)); // ["hello hello", "hello"]